I. Field of the Invention
This invention relates to photonic amplifiers and especially to photonic amplifiers which are intended for use in a long series of amplifiers.
II. Related Art and Other Considerations
Photonic amplifiers are used in telecommunications, e.g. in optical submarine cable systems, and it is desirable that such systems should include provision to indicate the probable location of certain faults. Specifically it is desirable to provide for the approximate location of a broken link which, of course, completely terminates any communication across the break. Clearly, a break implies that signal levels fall to zero. In addition to the breakage of a link, a similar condition occurs due to the deterioration of components. Thus, the failure of an amplifier would be equivalent to a break, and the deterioration of an amplifier would result in loss of gain whereby signal levels might fall to an unacceptably low level. Conditions such as the total loss of signal would probably be noticed in any system. However the location of the fault may not be apparent in a larger system and it is the object of this invention to provide information which helps to locate a fault.
A photonic amplifier amplifies optical signals using laser action, that is, signal photons stimulate the emission of similar photons whereby the signal is amplified. It is convenient to distinguish between two types of photonic amplifier by reference to the nature of the lasing medium. In the first type, the lasing medium is a semiconductor and the amplifier has a complicated structure involving several functional regions and interfaces. In the case of a semi-conductor amplifier, the power to drive the amplification is provided by an electric current from a suitable electronic circuit. In the second type of amplifier, the lasing medium is an optical fibre which is doped with a lasing species such as Nd or Er. In the case of a fibre amplifier, the power for the amplification is provided in the form of pump radiation which may have a shorter wavelength than the signal radiation. The pump radiation produces a population inversion in the lasing species in accordance with the well known theory for the operation of lasers.